MANCHESTER, Iowa — More than $1 million in negative advertising — much of it bankrolled by Mitt Romney's allies — has eroded Newt Gingrich's standing in Iowa and thrown the Republican presidential race here wide open two weeks before the first votes.

The former House speaker's Iowa slide mirrors his newfound troubles nationally, and it has boosted Romney's confidence while fueling talk that libertarian-leaning Texas Rep. Ron Paul could pull off a win in the first state caucuses Jan. 3.

"It's very disappointing to see so many of my friends who are running put out such negative junk, " Gingrich said Monday as he arrived in Davenport, poking at his opponents even as he insisted he was running an upbeat campaign. "I really wish they would have the courage to be positive."

Despite his chiding, attacks against him are all but certain to continue. For one, the Restore Our Future political action committee, made up of former Romney staffers from his failed 2008 bid, plans to spend $1.4 million more over the next two weeks, including on a new ad beginning Tuesday that's expected to be aimed at Gingrich. That would bring to roughly $3 million the amount spent by the group against Gingrich.

Aides for several campaigns competing against Gingrich as well as outside independent groups aligned with the candidates say their internal polls find that he has fallen over the last week from the top slot in Iowa. And a national Gallup poll released Monday found Gingrich's support plummeting: He had the backing of 26 percent of Republican voters nationally, down from 37 percent Dec. 8. Romney's support was largely unchanged at 24 percent.

Gingrich's weakened position follows a barrage of advertising that cast him as a longtime Washington, D.C., power broker. The ads, primarily financed by so-called super PACs, underscore the power of independent groups following a Supreme Court decision last year that allowed people, unions and corporations to donate unlimited amounts of money to outfits advocating the election or defeat of candidates. Since the ruling, groups have popped up to work on behalf of every serious Republican presidential candidate.

With the caucuses looming in two weeks, the race in Iowa is arguably anyone's to win. And the results here will shape the rest of the state-by-state march to the GOP nomination.

Gingrich has acknowledged that the onslaught has taken a toll and tested his pledge to keep his criticism focused on Democratic President Barack Obama.

The Republican rushed back to Iowa on Monday after a three-day absence for several days of campaigning before voters tune out this weekend for the Christmas holiday.

He told about 200 people in the garage of a security company in Davenport that he would launch a 44-stop jobs and prosperity tour before the caucuses, and use those events to answer any charges put out there.

Gingrich also redoubled his appeals to conservatives, who make up the base of the GOP, with sharp criticism of the judiciary, saying he would have the Justice Department instruct the U.S. Marshals Service to arrest judges who ignore subpoenas to testify in Congress about their decisions. And he tried anew to end accusations he lobbied on behalf of troubled Freddie Mac or other organizations.

"We should have had a much more coherent answer," he said about charges that he earned a windfall from the federally backed mortgage giant.

He then offered his latest explanation, saying that his consulting firm, the Gingrich Group, was hired over a period of six years for strategic advice and he earned about $35,000 a year — "less than I got per speech." Gingrich said that when Freddie Mac was seeking a bailout in 2008, he told House Republicans "my position was to not give them money." Altogether, Gingrich's firm earned some $1.6 million from Freddie Mac.

As Gingrich tried to answer the criticism, Romney, his chief rival, was largely laying low, increasingly expressing optimism as he revels in a series of endorsements from establishment GOP figures such as Bob Dole, the 1996 GOP nominee, early state leaders such as South Carolina Gov. Nikki Haley, and newspapers including The Des Moines Register.

Paul, who has built arguably the largest get-out-the-vote organization in Iowa and has steadily been inching up in Iowa polls, spent the day in New Hampshire before returning to Iowa for a packed schedule later in the week. He's been on the air in Iowa with ads assailing Gingrich.

Former Pennsylvania Sen. Rick Santorum was among several conservatives canvassing Iowa in hopes of taking advantage of Gingrich's slide and mounting a late-game surge.

Another, Texas Gov. Rick Perry, was in the midst of a bus tour when he slapped at two strong-running candidates Monday over their past support of the $700 billion Wall Street bailout while visiting a pizza buffet in Manchester.

"This Wall Street bailout is the single biggest act of theft in American history," he said. "And, you know, Newt and Mitt, they both were for it. That's one of the reasons I say that if you really want an individual who is an outsider, someone who has not been engaged in part of that process, I hope you'll take a look at me."

Most of the money lent to the financial institutions has been repaid.

On her own bus tour of the state, Minnesota Rep. Michele Bachmann, looking to peel off Paul supporters, sought to sow doubt about Paul's opposition to pre-emptive military action in nations such as Iran and North Korea.

"Ron Paul would be a dangerous president," Bachmann said in Grundy Center. "He would have us ignore all of the warning signs of another brutal dictator who wants to wipe Israel off the face of the earth. I won't."

Gingrich, indirectly but unmistakably, went after Paul, too, for wanting to close U.S. military bases abroad and bring all or nearly troops home. "I stand apart from some of our candidates in believing we need a strong defense," Gingrich asserted.

That criticism aside, the vast majority of attacks over the past week have been against Gingrich, and not limited to television advertising.

An anonymous independent group calling itself Iowans for Christian Leadership is urging conservatives not to back Gingrich, in light of his two divorces and past marital infidelity. The group has issued fliers and posted a scathing online video aimed at Gingrich, but has not begun showing TV ads.

The pro-Romney group, meantime, has spent $1.1 million on Iowa advertising over the past two weeks with a spot referring to Gingrich's "baggage," including ethics charges that led to his departure from Congress.

Paul's campaign has also run an ad pointedly attacking Gingrich's work for Freddie Mac and his former support for a health care mandate, a position unpopular with conservatives. And Perry also has started to run ads against Gingrich.

All have painted Gingrich as a Washington insider who profited from his stature after leaving Congress more than a decade ago.

Paul is scaling back his advertising to $55,000 or so over the next two weeks but the pro-Romney super PAC is filling the void with roughly $1.4 million in ad time reserved for the rest of the Iowa campaign.

The group also is advertising in Florida, spending a modest amount, roughly $143,000 over two weeks. But the ad buy is significant because Florida, which holds its primary Jan. 31, is seen as a potential showdown for Romney and Gingrich.